This invention relates to a data transmission system, which permits data transmission and reception among data processors connected to a communication medium via interfaces.
A fully distributed peer protocol transmission system or CSMA/CD (carrier sense multiple access with collision detection) system is well known in the art as a bus-shaped network, which does not require any central control station and is readily capable of extension. The system effects baseband transmission via a coaxial cable as a communication line. Recently, in order to improve the efficiency of utility of the coaxial cable, broadband networks are being developed, in which a baseband signal is converted into a carrier transmission signal.
When realizing a CSMA/CD system on a carrier transmission line, it is significant to ensure collision detection characteristics substantially comparable with those on the conventional baseband transmission line and also ensure compatibility with upper protocols.
To meet collision detection characteristics requirements, there are some approaches.
In a first approach, transmitted data is temporarily stored and collated bit by bit with returned data via a transmission line. If the two data are identical for all bits, it is assumed that the data has been transmitted without collision. If the two data differ even for a single bit, it is assumed that a collision has occurred. This system is referred to as bit collation system of transmitted and received data.
In this system, however, it is necessary to demodulate and decode the received signal to obtain received data and also store the transmitted data. This means that, in order to ensure the compatibility with upper protocol, part of functions of upper protocol must be effected by physical levels of lowest protocols. This will complicate the construction and increase cost of a modem as hardware which realizes the physical level. Another peculiar problem of this system is that only a transmitting modem can detect a collision and a receiving modem cannot. Therefore, while a modem is executing a random process called "backoff" after the detection of a collision, another modem which has been in the receiving condition may be entitled to data transmission. This will lead, under a high traffic condition of the network, to a troublesome situation in which a modem which has once encountered a collision will be unable to transmit data indefinitely. Further, the system cannot determine how many collisions occur while monitoring the traffic of the network. This poses a problem in connection with the control of network.
In a second approach, a modem which intends to transmit data sends out two pulses at a random time interval prior to data transmission and monitors the transmission line for a period of time two times the maximum propagation time of the transmission line from the point of time at which the first pulse is sent out. If only two pulses are received during the period of time, it is assumed that no collision has occurred so that data transmission is commenced. This system is called "random pulse monitoring system".
This system, however, requires the time of monitoring for any collision every transmission of a data packet. This reduces the transmission efficiency, i.e., effective transmission capacity, of the network. Besides, like the first system, to ensure the compatibility with upper protocols, the modem is required to store data to be transmitted from the upper protocol while monitoring two random pulses. This complicates the hardware of modems.
In a third approach, the detection of a collision is done using a fact that a peak level of a beat signal resulting from an overlap of two signals transmitted from different modems is doubled when the two signals are in phase, for instance. This system is called "signal level detection system". This approach is free from the drawbacks in the previous first and second approaches, i.e., the incapability of collision detection by a receiving modem and the reduction of the network efficiency.
In this system, however, signals transmitted from all the other modems must be received through the transmission line by any modem at an equal and stable level. If the levels of the received signals are different, it becomes difficult to decide the occurrence of a collision.
From the standpoints of the signal transmission quality and collision detection in the CSMA/CD system, it is very important to make equal the levels of signals received by one interface from all the other interfaces.
However, with such modulation systems as VSB and duobinary AM/PSK systems which are currently attracting attentions in various aspects, the carrier frequency is not essentially varied. Therefore, the beat signal frequency resulting from a collision of signals is very low, and there is no quarantee that the double peak level due to overlap of in-phase signals is always obtained during the period during which a data packet is transmitted. This means that 100% reliable detection of collisions cannot be ensured.